In order to prevent the unlawful copying and falsifying of image information, extensive research has been conducted with a view to embedding specific information in this image information. Means for achieving this is referred to as an electronic watermark. For example, a known technique is to embed additional information in image information obtained by electronically digitizing photographs and pictures, wherein the additional information is the name of the copyright holder or an indication as to whether use of the image is allowed or not. A technique that has been standardized in recent years involves embedding additional information in original image information in such a manner that the additional information is not noticeable visually, and distributing this image information over a network such as the Internet.
Another technique being studied involves arranging it so that additional information can be identified from the paper on which an image has been printed, wherein the additional information is the type of the printer that printed the image, the model number of the printer, etc. Improvements in the image quality of image forming devices such as copiers and printers have been accompanied by use of this technique for the purpose of preventing the counterfeiting of banknotes, stamps and securities, etc.
By way of example, the specification of Japanese Patent Application Laid-Open No. 7-123244 proposes a technique for embedding additional information in that part of an image of low visual sensitivity in which color-difference and saturation components are in the high-frequency region. Further, the specification of Japanese Patent No. 2614369 proposes a technique for embedding additional information in image information by adding on a specific dot pattern using a color that is comparatively difficult for the human eye to distinguish, such as the color yellow.
Though the conventional methods described above are effective in terms of embedding a comparatively small quantity of information for preventing counterfeiting, such as the identification number of the printer, the printing date and user information, in an image, it is very difficult for these methods to embed voice information and other information of large quantity in an image so as not be noticeable at the time of printing.
Accordingly, in the specification of Japanese Patent Application Laid-Open No. 2001-148778, the present applicant has proposed means for solving this problem, namely a method of utilizing texture, which is produced by the error diffusion method, to artificially create a combination of quantization values not generated in ordinary quasi-continuous tone processing, and embedding the created code in image information. In accordance with this method, the form of the texture changes microscopically and therefore image information after it has been embedded with the additional information exhibits no visual decline in image quality when compared with the original image. Further, the multiplexing of signals of different types can be achieved very easily by changing the quantization threshold value used in the error diffusion method.
Accordingly, an image processing system proposed previously by the present application will now be described. The system includes an image processing apparatus for embedding additional information in image information and printing the result, and an image processing apparatus for extracting the embedded additional information from the printed image. FIG. 12 is a block diagram illustrating the structure of the image processing apparatus that embeds additional information in image information and prints the result.
As shown in FIG. 12, multitone image information D4 enters from an input terminal 121. Additional information x2(i), which is to be embedded in the image information D4, enters from an input terminal 122. Various information is applied as the additional information x2(i), namely information having absolutely no relation to the image information D4 and information relating to copyright of the image information D4.
The additional information x2(i) that has entered from the input terminal 122 is applied to an error correction encoder 123 having a function for automatically correcting a bit error. The encoder 123 applies encoding processing to the information to obtain an error correction code. A block code such as BCH code or Reed-Solomon code or a code such as a convolution al code is used as the error correction code. Information obtained as a result of subjecting the additional information x2(i) to encoding processing for conversion to an error correction code shall be referred to as multiplexed information y2(j).
The image information D4 and the multiplexed information y2(j) enter an additional-information multiplexer 124. The latter divides the image information D4 into small square blocks in such a manner that the multiplexed information y2(j) to be embedded in the image information D4 will not be noticeable visually at the time of printing, and embeds the multiplexed information y2(j) on a per-block basis. The additional-information multiplexer 124 applies quantization to the image information obtained by embedding the multiplexed information y2(j) in the image information D4. Image information D5 obtained through quantization by the additional-information multiplexer 124 is printed on paper by a printer 125, whereby a printed image 126 is obtained. The printer 125 used is a printer such as an inkjet printer or laser printer for expressing tones by using quasi-continuous tone processing.
FIG. 13 is a block diagram illustrating the structure of a previously proposed image processing apparatus for reading and extracting additional information from a printed image. As shown in FIG. 13, the printed image 126 that has been printed by the image processing apparatus of FIG. 12 is read using an image scanner 131, whereby image information D6 is obtained. Next, embedded multiplexed information y2′(j) is separated from the image information D6 by the operation of an additional-information demultiplexer 132. The latter divides the image information D6 into square blocks and analyzes the frequency components of texture on a per-block basis, thereby separating the embedded multiplexed information y2′(j) . An error correction decoder 133 subjects the separated multiplexed information y2′(j) to error correction decoding processing, whereby additional information x2(i) is acquired. The additional information x2(i) is output from an output terminal 134. Algorithms relating to multiplexing and decoding are described in the specification of Japanese Patent Application Laid-Open No. 2001-148778 and therefore are not discussed here. By applying the above-described method, it is possible to embed a large quantity of additional information in a certain image without degrading the quality of this image as compared with the conventional method.
In a case where additional information is embedded in a print image using the method disclosed in the specification of Japanese Patent Application Laid-Open No. 2001-148778, reading errors tend to occur in regions where density is extremely high, regions where density is extremely low, or regions that contain main frequency components that resemble the frequency of the texture used to express the additional information. The reason for this is that these regions are often concentrated in a specific area of the image. Hence there is a tendency for reading errors to concentrate in such a specific area. In the event of the occurrence of a burst error, which is such an error in a specific area, it is necessary to raise the error correction capability greatly if use is made of an encoding method, such as BCH encoding, in which encoding processing is executed in units of a fixed code length. As a consequence, there is a relative decrease in the quantity of information that can be embedded.
Means for dispersing reading errors is an effective solution to this problem. However, the conventional techniques do not provide means for shuffling, in which after a printed image is read using an optical reading device, reading errors that occur when the additional information is reconstructed block by block are dispersed.